The present invention relates to a magnetometer measuring head and to a magnetometer comprising such a head.
U.S. Pat. No. 4,112,367 discloses a magnetometer utilising the Faraday effect in thin layers of particular materials such as Yttrium and iron garnet commonly called YIG. In this magnetometer, a light beam produced by a laser is propagated in a thin layer having a thickness of a few micrometers. This beam is propagated in guided manner according to a single mode, for example the transverse magnetic mode (TM). To select this mode, and taking into account the small thickness of the layer, this beam has to be made to penetrate into this layer by means of a coupling prism. Under the action of the magnetic field permeating the thin layer, the propagation mode is partially converted into the transverse electric mode (TE). For extracting the beam from the layer, use is made of a prism of birefringent material supplying two angularly separated beams; the one corresponding to the TE mode and the other to the TM mode. The strength of the beam corresponding to the TE mode is a function of the strength of the magnetic field permeating the thin layer. Since this is not a linear function, a zeroing method is utilised to perform the measuring operation, which consists in compensating within the thin layer of the field which is to be measured by an opposed adjustable field generated by a coil. To detect this compensating action, this direct field has superimposed on it an alternating field which within the beam corresponding to the TE mode causes the appearance of a modulation which when the balancing operation is effected, contains only even harmonics of the frequency of the alternating field since the Faraday effect depends on the value of the magnetic field and not on its direction. If the compensation is not carried out, odd components and in particular the fundamental component, are detected in the modulated beam and these components are utilised to control a servo mechanism which adjusts the compensating field so as to secure the disappearance of these odd components.
Although the measuring layer is of small size, the associated elements needed to utilise the same are much bulkier and delicate to adjust. Furthermore, the single-mode propagation requires a high-power laser and thus one of large size as a light source. Finally the angular separation of the beams coming from the output prism is not very great and requires the detector to be positioned sufficiently far away to be energised only by the single beams selected.
In order to overcome these prior art disadvantages, a special system is described in French patent application No. 2,471,608, which corresponds to U.S. Pat. No. 4,449,096, enabling to obtain a head of small size able to measure the magnetic field in narrow spaces and enabling the plotting magnetic gradient charts. To this end, the magnetic material thickness is selected so as to allow multimode guide propagation of the beam and, in cooperation with the beam transmission mans and the rotation measuring means, the entry and exit of the beam via the edge of the layer.
Whilst retaining the advantages of this device, the present invention proposes a modified measuring head structure simultaneously increasing the useful mensuration distance, simplifying the mounting of the head and reducing its dimensions.